Moving on the Y axis - wont move 'up'

Hello all, I'm pretty new to unity. But I managed to find some code in the unity manual on getting characters to move. I adjusted it so I could move up and down the Y axis on keys 'q' and 'e'. The down works great but the up dos'nt work at all. Also when I use combinations of forward and straif I go down.

Theres somthing I've just not understood here, any help appriciated!

 using UnityEngine;
 using System.Collections;
 
 public class VehicleController : MonoBehaviour {
     public float speed = 10.0F;
 
     private Vector3 moveDirection = Vector3.zero;
     void Update() {
         CharacterController controller = GetComponent<CharacterController>();
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), Input.GetAxis("UpDown"), Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
             
         controller.Move(moveDirection * Time.deltaTime);
     }
 }